Fire plays an important role in determining structure of forests and scrublands throughout the West. Distribution and structure of vegetation depends upon topography, climatic regime, and fire regime. Six fire regimes are defined based on fire frequency and intensity, varying from frequent, low-intensity surface fires to very long return interval, stand replacement fires. In certain western forests and scrub- lands fire suppression for the past 50 to 100 years has led to longer intervals between fires, increases in surface and crown fuels, changes in forest structure, and sequential impacts on fire intensity, postfire age structure, species composition, fuel accumulation, and both horizontal and vertical pattern. Better understanding of fire regimes is basic to our management of western ecosystems.